Solar collector

ABSTRACT

A solar collector is made primarily of plastic materials. A wide variety of collector configurations may be made from similar parts, and thus the collector may be adapted to mount in locations where traditional flat panel collectors may not be feasible. The collector may conveniently be used with a nanofluid as the heat transfer fluid, to increase the heat transfer characteristics of the heat transfer fluid. A control system for stagnation remediation, freeze protection, or both may be utilized. For example, when the collector is in danger for stagnation or freezing, water may be circulated through a ground coupled heat exchange loop to cool or heat the collector. Preferably, the stagnation remediation mode does not sacrifice thermal energy previously collected and stored.

BACKGROUND OF THE INVENTION

Sunlight provides a source of renewable, clean, and freely available energy useful in a variety of applications, including water heating, space heating, electricity generation, and other areas. Devices for collecting and concentrating solar energy have been developed to take advantage of the ready availability of this energy source. For example, flat panel collectors are often used in low temperature applications such as space heating and domestic water heating. Concentrating collectors may be used in higher temperature applications such as electric power generation, industrial process heating, and other applications.

Conventional flat panel collectors have a number of disadvantages. For example, they are typically made of relatively expensive materials such as copper, aluminum, and glass due to requirements for strength and thermal conductivity. Flat panel collectors are also typically provided in relatively large form factors, for example up to four by eight feet, and thus may be difficult to integrate into architectural designs.

BRIEF SUMMARY OF THE INVENTION

According to one aspect, a solar energy collector comprises an elongate plastic receiving tube configured to carry a heat transfer fluid, and an elongate clear plastic tubular sheath surrounding the receiving tube. The sheath is of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath. The sheath has a front side configured to be disposed toward the sun and a back side opposite the front side. The solar energy collector further includes a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side of the tubular sheath and configured such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. In some embodiments, the receiving tube is made of black polyethylene tubing. The sheath may be made of clear polycarbonate tubing. In some embodiments, both the receiving tube and the sheath have circular cross sections.

According to another aspect, a solar energy collector includes a plurality of elongate plastic receiving tubes configured to carry a heat transfer fluid. The solar energy collector also includes a plurality of elongate clear plastic tubular sheaths, each sheath surrounding a respective one of the plurality of receiving tubes, each sheath being of a larger cross section than its respective receiving tube such that a generally annular air space is formed between sheath and the receiving tube. Each sheath has a front side configured to be disposed toward the sun and a back side opposite the front side. A portion of the interior surface of the sheath at the back side is covered by a reflective coating configured such that when sunlight is directed to the uncoated portion of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly and second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. The solar energy collector further includes an inlet manifold including an inlet opening for receiving the heat transfer fluid to be heated and a plurality of outlet openings, and an outlet manifold including a plurality of inlet openings and an outlet opening. Each of the plurality of receiving tubes is coupled between one of the plurality of outlet openings of the inlet manifold and one of the plurality of inlet openings of the outlet manifold. In some embodiments, the inlet and outlet manifolds are made of plastic. In some embodiments, the plurality of receiving tubes are disposed parallel to each other to form a rectangular collector unit. In some embodiments, the inlet manifold, the outlet manifold, the plurality of receiving tubes, and the plurality of sheaths are comprised in a first collector unit, and the solar energy collector comprises one or more additional collector units of like construction to the first collector unit, the inlet manifolds of the one or more additional collector units operatively coupled to the inlet manifold of the first collector unit and the outlet manifolds of the one or more additional collector units operatively coupled to the outlet manifold of the first collector unit. In some embodiments, gaps exist between adjacent members of the plurality of sheaths. The solar energy collector may have an aspect ratio of at least 3:1. The solar energy collector may have an aspect ratio of at least 5:1.

According to another aspect, a solar energy collection system includes a solar energy collector comprising an elongate plastic receiving tube surrounded by an elongate clear plastic sheath. The sheath is of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath. The sheath has a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. The solar energy collection system further includes a supply of the heat transfer fluid to be heated by the solar energy collector. In some embodiments, the solar energy collector comprises a plurality of elongate plastic receiving tubes, each surrounded by a respective elongate clear plastic sheath, the plurality of sheaths and receiving tubes are arranged in parallel such that the solar energy collector is rectangular, and the solar energy collector comprises an inlet manifold and an outlet manifold to direct the heat transfer fluid through the parallel receiving tubes. The heat transfer fluid may be water comprising nanoparticles. The solar energy collection system may further include an ion generator to generate the nanoparticles. In some embodiments, the solar energy collection system further includes a tank for holding the supply of heat transfer fluid, a supply pipe for carrying heat transfer fluid from the tank to the solar energy collector, a return pipe for carrying the heat transfer fluid from the solar energy collector to the tank, a circulation pump for circulating the heat transfer fluid between the solar energy collector and the tank through the supply pile and the return pipe, and a controller that controls the operation of the circulation pump based at least in part on the temperature of the heat transfer fluid in the tank and the temperature of the solar energy collector. The controller may be configured to determine when the solar energy collector reaches a stagnation condition and in response to the determination, enter a stagnation remediation mode. In some embodiments, the solar energy collection system further includes a source of cooling fluid, wherein during the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector without passing through the tank. In some embodiments, the solar energy collection system further includes a ground coupled heat exchanger, wherein in the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector and the ground coupled heat exchanger without passing through the tank. In some embodiments, the ground coupled heat exchanger comprises a piping loop connected between the supply pipe and the return pipe, and the system further comprises a set of valves operated by the controller to isolate the tank during the stagnation remediation mode. In some embodiments, the solar energy collection system further includes a photovoltaic panel that supplies power to operate the controller and the circulation pump. In some embodiments, the solar energy collection system further includes a root zone heating loop, wherein the system circulates the heat transfer fluid through the root zone heating loop to heat the root zone of plants.

According to another aspect, a method of collecting solar energy includes providing a solar energy collector. The solar energy collector includes one or more elongate plastic receiving tubes each surrounded by a respective elongate clear plastic sheath. Each respective sheath is of a larger cross section than its respective receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. The method further comprises installing the solar energy collector in a location that receives sunlight, and passing a heat transfer fluid through the solar energy collector to be heated by the sunlight. In some embodiments, the solar energy collector has an aspect ratio of at least 3:1, and installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio. In some embodiments, the solar energy collector has an aspect ratio of at least 5:1, and installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a collector unit, in accordance with embodiments of the invention.

FIG. 2 shows a partially exploded view of one end of the example collector unit of FIG. 2, and illustrates some additional details of the construction of the collector unit of FIG. 1.

FIG. 3 shows a sectional view of the collector unit of FIG. 1, and illustrates additional aspects of its operation.

FIG. 4 illustrates other examples of receiving tubes and sheaths according to embodiments of the invention.

FIG. 5 illustrates combining several collector units like the collector unit of FIG. 1 into a larger collector, in accordance with embodiments of the invention.

FIG. 6 illustrates a system including an installation of the collector of FIG. 5 on a roof, used for heating water in accordance with embodiments of the invention.

FIG. 7 illustrates a system for solar heating of domestic hot water, in accordance with embodiments of the invention.

FIG. 8 illustrates a system that uses a ground coupled heat exchanger to provide cooling fluid to a collector, in accordance with embodiments of the invention.

FIG. 9 shows a collector in accordance with another embodiment of the invention.

FIG. 10 illustrates a solar energy collection system adapted for root zone heating, in accordance with embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention provide a solar collector made of low-cost, lightweight polymer materials.

FIG. 1 illustrates a collector unit 100, in accordance with embodiments of the invention. Collector unit 100 includes an inlet manifold 101 and an outlet manifold 102. During use of collector unit 100, light from the sun 103 falls on collector unit 100. A heat transfer fluid enters inlet manifold 101 through inlet opening 104, travels through receiving tubes not visible in FIG. 1, where it is heated by the sun 103. The heat transfer fluid then exits outlet manifold 102 through outlet opening 105. Some of the heat transfer fluid may continue through inlet manifold 101 to one or more additional collector units similar to collector unit 100, as will be described in more detail below.

FIG. 2 shows a partially exploded view of one end of example collector unit 100, including outlet manifold 102, and illustrates some additional details of the construction of collector unit 100. Outlet manifold 102 is preferably made of injection molded plastic, for example a polymer such as polycarbonate, nylon, ABS, or another suitable polymer. The polymer may be reinforced, for example with glass fibers. Besides outlet opening 105, outlet manifold includes four side tubes 201 (only one of which is visible in FIG. 2), and another end opening 202. The particular configuration of outlet manifold 102 may also be called a “multi-port tee”. Each side tube and the ends of outlet manifold 102 include fittings for joining tubes, caps, or other item. In the example of FIG. 2, simple barb fittings are shown, but other kinds of fittings could be used. For example, outlet manifold 102 could be a 4-outlet, ¾ inch molded flow-through multi-port tee available from Uponor North America of Apple Valley, Minn., USA, and having fittings for attaching PEX tubing. Any other suitable kind of manifold may be used, for example a manifold having a different number of openings. In some embodiments, metal manifolds may be used.

Collector unit 100 also includes four elongate receiving tubes 203, one for each of side tubes 201. Although other materials and sizes may be used, receiving tubes 203 may be made of flexible ½ inch black polyethylene pipe, which is manufactured in lengths up to hundreds of feet, and is typically packaged in rolls. The black color of the pipe may facilitate absorption of solar energy. Receiving tubes 203 may be connected to outlet manifold 102 in any suitable way, for example using crimp clamps 204 (only two of which are visible in FIG. 2).

Collector unit 100 further includes four elongate clear plastic tubular sheaths 205, one surrounding each receiving tube 203. Sheaths 205 are a form of glazing and may be made, for example, of clear polycarbonate tubing or another suitable material that is transparent to sunlight or nearly so. In the example of FIG. 2, sheaths 205 may have a circular cross section with a diameter of about 1.25-1.5 inches, although other sizes may be used in other embodiments. One side of the inside surface of each sheath 205 is coated with a reflective coating 206. Reflective coating 206 may be made, for example, of an adhesive-backed aluminum foil, aluminized Mylar, or another suitable reflecting material. Insulating spacers 207 may be fitted over receiving tubes 203, to maintain the spacing of sheaths 205 to their respective receiving tubes 203, and to provide an insulating function. Insulating spacers 207 may be made, for example, of a urethane foam or another suitable material. While insulating spacers 207 are shown in FIG. 2 only at the ends of sheathes 205, additional spacers could be placed at other locations along the length of collector unit 100.

FIG. 3 shows a section view of collector unit 100, and illustrates additional aspects of its operation. As is apparent in FIG. 3, some of sunlight 301 striking clear sheath 205 passes through clear sheath 205 and strikes receiving tube 203 directly. Another portion of the light strikes reflective coating 206 and is redirected to strike receiving tube 203. The sheaths 205 thus perform multiple functions. They lend structure and stiffness to collector unit 100, by virtue of their larger diameter than receiving tubes 103. Sheaths 205 also serve to reduce heat loss from receiving tubes 103, protecting receiving tubes 103 from breezes that would remove heat, and forming a generally annular air space 302, which may further insulate receiving tube 103. In addition, sheaths 105 support reflective coating 206, which effectively increases the effective collection area of collector unit 100, as compared to a collector lacking reflective coatings 206.

In FIGS. 1-3, receiving tubes 203 and sheaths 205 are shown as circularly cylindrical perfectly co-axial, but this is not a requirement. Tubes and sheaths having other cross sectional shapes may be utilized, for example elliptical, polygonal, or other shapes. For example, a sheath having a parabolic trough shape on its underside may be used. In another example, a shape including an involute of the collector tube may be used. However, in some embodiments cylindrical sheaths may be preferred for simplicity, cost, or other reasons. Polycarbonate tubes designed for protection of fluorescent light bulbs may be used in some embodiments, and are readily available at low cost.

In FIGS. 2 and 3, the spaces between receiving tubes 203 and sheaths 205 are annular, being the space between two co-axial cylinders, but this is not a requirement, and in other embodiments, the space between the receiving tube and sheath may be only generally annular. The term “generally annular” is intended to encompass the arrangement as shown in FIG. 3, where receiving tubes 203 and sheaths 205 are perfectly cylindrical and perfectly co-axial, such that air space 302 is truly annular in the mathematical sense, but also to encompass spaces defined when the sheaths surround the receiving tubes but are not perfectly co-axial, and spaces defined when the receiving tube, sheath, or both are not cylindrical.

FIG. 4 illustrates other examples of receiving tubes 401 a-401 d that are surrounded by sheaths 402 a-402 d, forming generally annular spaces 403 a-403 b between receiving tubes 401 a-401 d and sheaths 402 a-402 d. Many other arrangements may be used within the scope of the appended claims.

FIG. 5 illustrates combining several collector units like collector unit 100 into a larger collector 500, in accordance with embodiments of the invention. Collector 500 includes three collector units, including collector unit 100, joined using at their inlet and outlet manifolds. Heat transfer fluid can enter at inlet opening 104, travel in parallel through the receiving tubes of collector 500, and exit at outlet opening 105 after having been heated by the sun 103. Collectors of other desired sizes can be constructed by joining other numbers of collector units 100. In the embodiment pictured, each collector unit uses receiving tubes and sheaths about four feet long, and spaced about 1.5 inches apart, so that each collector unit 100 is about six inches wide and about four feet long, and collector 500 is about 18 inches wide and about four feet long. Other sizes and spacings may be used as well as other numbers of collector units, to form a collector of any workable size.

In some embodiments, the width of each sheath is smaller than the spacing between the receiving tubes, so that a gap exists between adjacent sheaths. This gap may allow airflow through the collector, and may reduce the effect of wind on the collector.

FIG. 6 illustrates a solar energy collection system 600 including an installation of collector 500 on a roof 601, used for heating water in accordance with embodiments of the invention. In this simple installation, system 600 heats a supply of heat transfer fluid 602 in tank 603. Heat transfer fluid 602 is drawn out of tank 603 and pumped by circulation pump 604 through supply pipe 605 to collector 500. Heat transfer fluid 602 is heated by solar energy in collector 500, and flows out of collector 500 and back to tank 603 through return pipe 606. In this example, tank 603, collector 500, and the associated piping form a closed system. Heat transfer fluid 602 may be used as a heat reservoir to provide a heat source for thermoelectric power generation, hydronic heating, root zone heating of plants, or for other uses. Pipes 605 and 606, as well as the manifolds of collector 500 are preferably insulated to avoid heat loss, but no insulation is shown in FIG. 6 for clarity of illustration.

The use of a heat reservoir of this type for various purposes is discussed in co-pending U.S. patent application Ser. No. 12/481,745 filed Jun. 10, 2009 and titled “Integrated Energy System for Whole Home or Building”, the entire disclosure of which is hereby incorporated by reference herein for all purposes. Other related information may be found in U.S. patent application Ser. No. 12/481,741 filed Jun. 10, 2009 and titled “Thermoelectric Generator”, and U.S. patent application Ser. No. 12/481,750 filed Jun. 10, 2009 and titled “Automatic Configuration of Thermoelectric Generation System to Load Requirements”, the entire disclosures of which are hereby incorporated by reference herein for all purposes.

In one simple mode of operation, a controller 608 receives signals indicating the temperature of heat transfer fluid 602 in tank 603, and the temperature of collector 500. If heat transfer fluid 602 in tank 603 is not at its desired temperature, and collector 500 is at a temperature higher than the temperature in tank 603, controller 602 causes circulation pump 604 to run so that the available energy at collector 500 is used to heat tank 602. In some embodiments, a photovoltaic panel 612 may be used to power controller 608 and circulation pump 604.

In some embodiments, heat transfer fluid 602 is water. Because collector 500 is mounted on roof 601 and exposed to the elements, water in collector 500 could be subjected to freezing temperatures. Preferably, tank 603 is located in a conditioned space, and system 600 operates as a “drain back” system. A volume of air may be included in the system, sufficient to fill collector 500 and the portions of pipes 605 and 606 that are above roof 601 and subject to freezing. When circulation pump 604 is not running, for example when the collector temperature is very low due to cold weather and is lower than the temperature of tank 603, heat transfer fluid 602 can drain back into tank 603 by the action of gravity, so that collector 500 and the upper portions of pipes 605 and 606 fill with air, and freezing of water in collector 500 is prevented. Preferably, collector 500 is mounted in such a way that corner 609 is the lowest portion of collector 500, so that heat transfer fluid 602 can drain as completely as possible from collector 500 by gravity.

Optionally, a vacuum breaker valve may be placed collector 500, for example at topmost corner 613, to admit air to collector 500 during drain back, to speed the drain back of heat transfer fluid 602.

Traditionally, solar collectors have been made of materials that are highly thermally conductive, for example aluminum and copper, in order to promote the efficient transfer of heat from the collectors to the heat transfer fluid used in them. However, collector 500 is made primarily of plastic materials, which have much lower thermal conductivity than traditional collector materials. For the purposes of this disclosure, to be made primarily of plastic materials means that the receiving, fluid distribution, and glazing components of the collector are made of polymer materials. Some metal parts may be used in a collector that is made primarily of plastic materials, for example for reflective coatings, clamps, mounting hardware, and other incidental functions. The primarily plastic construction of a collector in accordance with embodiments of the invention may afford advantages in material cost, shipping cost, and ease of assembly of the collector. In addition, the light weight of the collector may enable mounting lighter mounting hardware, and mounting the collector in locations that would not support a heavier collector made of traditional materials.

Even though made primarily of plastic, a model collector made according to an embodiment of the invention has proved surprisingly effective, especially in light of the low cost of its components. For example, a system similar to system 600 was constructed including a collector made the manner shown in FIGS. 1 and 2. The collector included four parallel receiving tubes 20 feet long, and thus had a collecting aperture totaling 10.73 square feet. The system was tested on three sunny days in late winter of 2013 in Longmont, Colo., to heat water in a tank. Performance was evaluated by measuring the temperature of the tank, operating the collector to heat the tank, again measuring the temperature of the tank, and calculating the amount of energy delivered to the water in the tank from the change in temperature and the mass of water. In three tests on three different days, the system delivered an average of 6684 BTUs of thermal energy to the tank, or 623 BTUs per square foot of collector aperture area. In each test, the system was operated for about 5.5 hours, and the outdoor temperature reached about 57-60° F.

The performance of a system embodying the invention can be further improved by enhancing the heat transfer characteristics of heat transfer fluid 602. One especially effective and economical technique for enhancing the thermal conductivity and convective heat transfer characteristics of water is to convert the water to a nanofluid having suspended nanoparticles. Techniques for generating nanoparticles in water are described in co-pending U.S. patent application Ser. No. 13/035,479 filed Feb. 25, 2011 and titled “Thermoelectric Generator Using Nanofluid”, and U.S. Provisional Patent Application No. 61/680,671 filed Aug. 7, 2012 and titled “Efficient Energy Collection and Distribution”, the entire disclosures of which are hereby incorporated by reference herein for all purposes.

In one embodiment as illustrated in FIG. 6, an ion generator 610 includes two sterling silver electrodes 611 that are suspended in heat transfer fluid 602. Electrodes 611 may be positioned in tank 603, in one of pipes 605 and 606, or at another convenient location in the system. It may be advantageous to place the electrodes at a location where heat transfer fluid 602 flows regularly. Ion generator 610 contains circuitry that impresses an alternating voltage between the two electrodes, causing the electrodes to shed ions by electrolytic action. The ions are nanoparticles. The flow of heat transfer fluid 602 distributes the nanoparticles throughout the system. The presence of nanoparticles significantly enhances the heat transfer characteristics of the water, and therefore improves the performance of the system.

The test results described above for the model system were obtained using pure water as the heat transfer fluid. The same system was tested again with the addition of nanoparticles, and its performance improved significantly. The system was operated in the same manner as described above, on three sunny days in late winter of 2013 in Longmont, Colo. When operated using water including nanoparticles as the heat transfer fluid, the system delivered an average of 8672 BTUs to the tank, or 808 BTUs per square foot of collector area. The addition of nanoparticles thus at least partially counters the effect of the lower conductivity of the collector materials.

Other arrangements are possible. For example, if it is desired to use a system similar to system 600 to heat domestic hot water (DHW), then the heat transfer fluid circulated through the collector may be isolated from the domestic hot water tank, using a heat exchanger to transfer heat to the domestic hot water tank. An example of such a system 700 is shown in FIG. 7. Several components of system 700 are similar to components in system 600, and are given like reference numerals in FIG. 7. System 700 includes a heat exchanger 701 to isolate the heat transfer fluid that flows through collector 500 from domestic hot water 702 in tank 603. Also, electrodes 611 of ion generator 610 are positioned in pipe 605, rather than in tank 603. Finally, a drainback tank 703 is provided, so the heat transfer fluid circulating to collector 500 can drain completely back into the conditioned space in times of freezing weather. Alternatively, the heat transfer fluid flowing through collector 500 could be treated with an anti-freeze additive, in which case the closed loop could be completely filled and not drain back tank would be necessary.

Another phenomenon that may be of particular interest in a plastic solar collector is stagnation of the collector. Stagnation refers to the condition where the heat transfer fluid is not circulating through the collector, but the collector is in sunlight. This situation may occur, for example, if tank 603 has reached its desired temperature and controller 608 shuts off pump, even though collector 500 is at a higher temperature than tank 603. Without the circulating heat transfer fluid to carry heat away from collector 500, collector 500 may reach such high temperatures that collector 500 may be damaged.

In conventional systems having collectors made of metal parts, simply draining back the heat transfer fluid may be sufficient to provide adequate protection, as the metal parts may be able to withstand the temperatures reached by the empty collector. However, the plastic materials used in a collector according to embodiments of the invention may have a relatively low melting temperatures, and may be subject to thermal expansion and warping whether filled or empty.

In some embodiments, protection from damage due to stagnation may be provided by circulating a cooling fluid through the collector when the storage tank is at its full temperature, but the collector is in sunlight. FIG. 8 illustrates a system 800 that uses a ground coupled heat exchanger in the form of piping loop 801 to provide cooling fluid to collector 500, in accordance with embodiments of the invention. Ground coupled piping loop 801 may be, for example, a length of plastic pipe buried in a trench a few feet deep, packed with sand, and covered with earth. The temperature of soil underground tends to remain very stable throughout the year, typically about 57° F. in many parts of the United States. The earth can therefore act as a large heat source or heat sink. When fluid is circulated through piping loop 801, the fluid gains heat from the ground if the fluid enters piping loop 801 at a temperature colder than the ground or gives up heat to the ground if the fluid enters piping loop 801 at a temperature warmer than the ground. Techniques for constructing piping loop 801 are given in U.S. Provisional Patent Application No. 61/680,671 previously incorporated by reference.

In system 800, controller 608 monitors the temperature of heat transfer fluid 602 in tank 603, and also the temperature of collector 500. When stagnation is detected, for example when tank 603 is at its maximum desired temperature and collector 500 exceeds a safe operating temperature, controller 608 enters a stagnation remediation mode. In one example embodiment, controller 608 may enter the stagnation remediation mode when collector 500 exceeds 120° F., although other criteria may be used.

In the stagnation remediation mode, circulation pump 604 is shut off. Preferably after a short delay, two normally-open valves 802 and 803 are switched closed under control of controller 608, to isolate tank 603. Second pump 804 is then started. Piping loop 801 has been previously filled with water, so that water is drawn by second pump 804 from piping loop 801, travels up pipe 605 to collector 500 where it is heated and cools the collector, and travels down pipe 606. Because normally-open valve 802 is closed, the water travels through pipe 805 back to piping loop 801 to give up heat and circulate for another cycle through collector 500 if needed. It is envisioned that only a few minutes of cooling will be required at any one time to bring collector 500 back to a safe temperature, and that the energy consumption of second pump 804 and valves 802 and 803 will therefore be small.

For added security, an additional, redundant pump 806 may be installed in parallel with second pump 804. Controller 608 may sense whether pump 804 is operating properly and if not, redundant pump 806 would be started, to ensure that collector 500 is properly cooled. In some embodiments, emergency battery power could be used to operate the system during power outages.

In other embodiments, for example for installations where it is not possible to install a collector in such a way that it can drain back completely, the stagnation remediation mode may also be used for freeze protection. That is, heat transfer fluid could be circulated through piping loop 801 and through the collector in times of freezing temperatures. Instead of cooling the collector to prevent damage from stagnation, the circulating fluid would be heated by the ground loop and would warm the collector to prevent damage from freezing. In some embodiments, no additional hardware would be necessary. The additional control logic could simply be programmed into controller 608.

While controller 608 is described above as being programmable, the system may also be implemented by a set of temperature sensors and logical connections between them, without requiring a programmable element such as a microprocessor. For example, a differential temperature sensor may compare the temperatures of the collector and tank, and generate a logic level indicating whether the collector is warmer or cooler than the tank. That signal would be routed to a drive circuit for circulation pump 604, and circulation pump 604 would run when the collector is warmer than the tank, but would not run otherwise. The drive signal could be further gated by other logic signals. For example, another differential sensor could compare the tank temperature to a desired upper limit value, and generate a logic level indicating whether the tank is at the desired temperature or not. If so, the drive signal to circulation pump 604 could be overridden so that circulation pump 604 would not run in this state. Other gating signals would be derived to indicate stagnation (collector above a stagnation limit temperature), freeze danger (collector below a threshold temperature), or other conditions. It is intended that the term “controller” encompass both programmable devices and control logic implemented by discrete components.

In one example embodiment, controller 608 may be programmed or otherwise configured to implement the following control scheme:

Valves 802 Pump Pump Pump and Condition 604 804 806 803 Comment a) tank below desired On Off Off Open Normal temperature heating b) collector temperature operation higher than tank temperature c) collector temperature below stagnation temperature a) tank below desired Off Off Off Open Normal temperature operation, b) collector not warmer wait for than tank collector c) collector not in to warm up danger of freezing a) tank at desired Off Off Off Open Normal temperature operation b) collector below when heat stagnation temperature not needed c) collector above freezing a) collector at or above Off On Off Closed Stagnation stagnation temperature remediation b) pump 804 working a) collector at or above Off X On Closed Stagnation stagnation temperature remediation b) pump 804 not working using redundant pump a) collector in danger Off On Off Closed Freeze of freezing protection b) pump 804 working using stagnation remediation mode a) collector in danger Off X On Closed Freeze of freezing protection b) pump 804 not working using stagnation remediation mode, redundant pump

While collector 500 shown in FIGS. 5-8 is in a rectangular configuration, the invention is not so limited, and it will be appreciated that the construction techniques and materials described above provide opportunities for configuration and customization of collectors, and may enable use of solar collectors in locations that were not previously convenient.

For example, FIG. 9 shows a collector 900 in accordance with another embodiment of the invention. Collector 900 includes a single elongate receiving tube 901 surrounded by a sheath 902. Receiving tube 901 may be made from a continuous plastic pipe, or may include splices. Similarly, sheath 902 may also include splices, and periodic spacers 903 may be used to maintain space between receiving tube 901 and sheath 902 if desired. Sheath 902 also preferably includes a reflective coating 904 on one side, to enhance the collection efficiency of collector 900, as described above. Collector 900 is depicted as being about 1.5 inches in diameter and about eight feet long, but the length of the collector is limited only by the practicality of installing it and the ability to pump heat transfer fluid through the narrow receiving tube for long distances. For example, collector 900 may have length as much as 50, 100, 200, 500, or more times its width. The ratio of the collector's length to its width may be called the collector's aspect ratio.

A collector such as collector 900 or another collector according to embodiments of the invention may be installed in places where a conventional collector panel could not. For example, collector 900 may be installed on top of a fence or wall, on a building ledge, on a building eave, or in another location that has good sun exposure but would not be convenient for mounting another kind of collector, for example a traditional flat panel collector having an aspect ratio of about 2:1. Collector 900 may be somewhat flexible, and could include right-angle or other angular bends through the use of simple and readily available fittings. Thus, solar energy may be utilized in an unobtrusive manner.

It may be especially helpful for such a system to include the stagnation remediation capability described above and shown in FIG. 8. A long narrow collector installed on top of a wall would likely not be configured to drain completely by gravity. The stagnation remediation mode may be especially useful with such an installation to avoid freezing on winter nights, for example.

Many other collector configurations are possible as well. For example, long narrow collectors having two, three, four, or another number of parallel receiving tubes and sheaths could be used, depending on the size and shape of the area available for mounting the collector. Such collectors may have aspect ratios much larger than the typical conventional flat panel collector, for example 3:1, 4:1, 5:1, 10:1, 20:1, 50:1, 100:1, 200:1, 500:1, or another aspect ratio, and may be installed in spaces that could not accommodate a collector of equal area having a significantly smaller aspect ratio. For the purposes of this disclosure, a significantly smaller aspect ratio is any aspect ratio less than about 50 percent of the aspect ratio of the collector in question.

As was mentioned earlier, one application of a solar energy collection system according to embodiments of the invention is root zone heating of plants. Research has shown that heating the soil in the root zone of plants can promote improved growth and productivity of the plants, and enables cultivation of plant species in much colder ambient environments than the plants' native environments. FIG. 10 illustrates a solar energy collection system 1000, adapted for root zone heating. Solar energy collection system 1000 is illustrated as similar to system 600 shown in FIG. 6, with the addition of a root zone heating loop 1001. A root zone heating pump 1002, under control of controller 608, circulates heat transfer fluid 602 from tank 603 through root zone heating loop 1001, delivering heat to roots 1003 of plants 1004. Controller 608 may control pump 1002 intermittently, for example on a fixed schedule, or based on the relative temperatures of tank 603 and the soil beneath plants 1004. Although not shown, root zone heating loop 1001 may be used in conjunction with stagnation remediation measures, for example as shown in FIG. 8.

The invention has now been described in detail for the purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the appended claims. It is intended that all workable combinations of the elements and features described herein are also considered to be disclosed. 

1. A solar energy collector, comprising: an elongate plastic receiving tube having an open first end and an open second end, the receiving tube configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end; an elongate clear plastic tubular sheath surrounding the receiving tube, the sheath being of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side; and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side of the tubular sheath and configured such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube.
 2. The solar energy collector of claim 1, wherein the receiving tube is made of black polyethylene tubing.
 3. The solar energy collector of claim 1, wherein the sheath is made of clear polycarbonate tubing.
 4. The solar energy collector of claim 1, wherein both the receiving tube and the sheath have circular cross sections.
 5. A solar energy collector, comprising: a plurality of elongate plastic receiving tubes each having a respective open first end and a respective open second end, and each of the plurality of receiving tubes being configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end; a plurality of elongate clear plastic tubular sheaths, each sheath surrounding a respective one of the plurality of receiving tubes, each sheath being of a larger cross section than its respective receiving tube such that a generally annular air space is formed between sheath and the receiving tube, each sheath having a front side configured to be disposed toward the sun and a back side opposite the front side, wherein a portion of the interior surface of the sheath at the back side is covered by a reflective coating configured such that when sunlight is directed to the uncoated portion of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly and second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube; an inlet manifold including an inlet opening for receiving the heat transfer fluid to be heated and a plurality of outlet openings; and an outlet manifold including a plurality of inlet openings and an outlet opening: wherein the outlet openings of the inlet manifold are coupled respectively to the first ends of the plurality of receiving tubes, and the inlet openings of the outlet manifold are coupled respectively to the second ends of the plurality of receiving tubes such that each of the plurality of receiving tubes is coupled between one of the plurality of outlet openings of the inlet manifold and one of the plurality of inlet openings of the outlet manifold.
 6. The solar energy collector of claim 5, wherein the inlet and outlet manifolds are made of plastic.
 7. The solar energy collector of claim 5, wherein the plurality of receiving tubes are disposed parallel to each other to form a rectangular collector unit.
 8. The solar energy collector of claim 7, wherein: the inlet manifold, the outlet manifold, the plurality of receiving tubes, and the plurality of sheaths are comprised in a first collector unit; and the solar energy collector comprises one or more additional collector units of like construction to the first collector unit, the inlet manifolds of the one or more additional collector units operatively coupled to the inlet manifold of the first collector unit and the outlet manifolds of the one or more additional collector units operatively coupled to the outlet manifold of the first collector unit.
 9. The solar energy collector of claim 7, wherein gaps exist between adjacent members of the plurality of sheaths.
 10. The solar energy collector of claim 7, wherein the solar energy collector has an aspect ratio of at least 3:1.
 11. The solar energy collector of claim 7, wherein the solar energy collector has an aspect ratio of at least 5:1.
 12. A solar energy collection system, comprising: a solar energy collector comprising an elongate plastic receiving tube having an open first end and an open second end, the receiving tube configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end, wherein the receiving tube is surrounded by an elongate clear plastic sheath, the sheath being of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube; and a supply of the heat transfer fluid to be heated by the solar energy collector.
 13. The solar energy collection system of claim 12, wherein: the solar energy collector comprises a plurality of elongate plastic receiving tubes, each surrounded by a respective elongate clear plastic sheath; the plurality of sheaths and receiving tubes are arranged in parallel such that the solar energy collector is rectangular; and the solar energy collector comprises an inlet manifold and an outlet manifold to direct the heat transfer fluid through the parallel receiving tubes.
 14. The solar energy collection system of claim 12, wherein the heat transfer fluid is water comprising nanoparticles.
 15. The solar energy collection system of claim 14, further comprising an ion generator to generate the nanoparticles.
 16. The solar energy collection system of claim 12, further comprising: a tank for holding the supply of heat transfer fluid; a supply pipe for carrying heat transfer fluid from the tank to the solar energy collector; a return pipe for carrying the heat transfer fluid from the solar energy collector to the tank; a circulation pump for circulating the heat transfer fluid between the solar energy collector and the tank through the supply pile and the return pipe; and a controller that controls the operation of the circulation pump based at least in part on the temperature of the heat transfer fluid in the tank and the temperature of the solar energy collector.
 17. The solar energy collection system of claim 16, wherein the controller is configured to determine when the solar energy collector reaches a stagnation condition and in response to the determination, enter a stagnation remediation mode.
 18. The solar energy collection system of claim 17, further comprising a source of cooling fluid, wherein during the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector without passing through the tank.
 19. The solar energy collection system of claim 17, further comprising a ground coupled heat exchanger, wherein in the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector and the ground coupled heat exchanger without passing through the tank.
 20. The solar energy collection system of claim 19, wherein the ground coupled heat exchanger comprises a piping loop connected between the supply pipe and the return pipe, and wherein the system further comprises a set of valves operated by the controller to isolate the tank during the stagnation remediation mode.
 21. The solar energy collection system of claim 16, further comprising a photovoltaic panel that supplies power to operate the controller and the circulation pump.
 22. The solar energy collection system of claim 16, further comprising a root zone heating loop, wherein the system circulates the heat transfer fluid through the root zone heating loop to heat the root zone of plants.
 23. A method of collecting solar energy, the method comprising: providing a solar energy collector comprising one or more elongate plastic receiving tubes, each of the one or more receiving tubes having a respective open first end and a respective open second end, and each of the one or more receiving tubes being configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end, each of the one or more receiving tubes being surrounded by a respective elongate clear plastic sheath, each respective sheath being of a larger cross section than its respective receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube; installing the solar energy collector in a location that receives sunlight; and passing a heat transfer fluid through the solar energy collector to be heated by the sunlight.
 24. The method of claim 23, wherein the solar energy collector has an aspect ratio of at least 3:1, and wherein installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio.
 25. The method of claim 23, wherein the solar energy collector has an aspect ratio of at least 5:1, and wherein installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio.
 26. The solar energy collector of claim 1, further comprising a plurality of insulating spacers disposed in the generally annular air space, for maintaining the spacing of the sheath from the receiving tube.
 27. The solar energy collector of claim 1, wherein the solar energy collector has only one receiving tube.
 28. The solar energy collection system of claim 18, further comprising: a first normally-open valve in the supply pipe and a second normally-open valve in the return pipe, wherein closing the first and second normally-open valves prevents flow of the heat transfer fluid into between the tank and the solar energy collector; a cooling loop containing the cooling fluid, the cooling loop joining the supply pipe and the return pipe; and a cooling pump; wherein when the first and second normally-open valves are closed, the cooling fluid can circulate through the cooling loop and the solar energy collector via the supply pipe and the return pipe, driven by the cooling pump. 